Conventional packerhead concrete pipe making machines are equipped with rotatable packerheads that are moved in an upward direction in molds to form concrete pipes. Conveyors are used to move concrete from hoppers into the molds. Manually operated controls have been used to regulate the speed of the conveyors to control the amount of concrete that is discharged into the molds. The manual control of the feeding of the concrete into the molds requires considerable experience to produce acceptable concrete pipe. Manual controls result in generally erratic packing conditions and nonuniform concrete packing throughout the length of the pipe. The manual controls are not susceptible to accurate termination of the concrete feed during the forming of the spigot end of the pipe. The spigots are soft with an insufficient supply of concrete or are overpacked with an oversupply of concrete.
An automatic concrete feed control for packerhead-type machines is disclosed in Fosse U.S. Pat. No. 3,619,872. This concrete feed control is responsive to the level of concrete accumulated above the packerhead in the mold. A sensing unit is used to sense the level of the concrete. The sensing unit includes a probe adapted to engage or otherwise determine when the top surface of the concrete is at a defined level above the packerhead. When the top surface of the concrete is thus sensed, an electrical switch is activated. The switch is part of the control circuit that regulates the operation of a motor that drives the concrete conveyor. The sensor is mounted on a stationary outer tube that supports the packerhead. The sensing units, while greatly improving the operation of packerhead pipe making machines, do present maintenance and operational problems.
For example, the consistency of the concrete mix fed into the molds varies from batch to batch. When a dry batch of concrete mix is fed into the molds, the packerhead can jam even though the level of the concrete remains the same. The level of dry concrete mixes must be lowered to achieve a desired compaction or concrete density. When the concrete mix is wet, a high level is needed to maintain the same packing action and concrete density. The concrete level sensing unit does not compensate for differences in the consistency of the concrete mix due to the amount of water as well as differences in aggregates used in the concrete mix.
An improved self-compensating concrete feed control system and method to maintain a substantially constant packing pressure and, hence, uniform wall density throughout the length of a pipe made on a packerhead machine are disclosed in Fosse, U.S. Pat. No. 4,340,553. The disclosed machine includes a counterrotating packerhead which includes upper and lower packerhead units and moves from the bottom to the top of a mold in a single pass operation to form a concrete pipe. A first motor is used to rotate the upper unit of the packerhead. The amount of concrete discharged into the mold is controlled in a manner to maintain a generally constant torque on the upper unit of the packerhead by sensing the power used by the first motor to rotate the upper unit of the packerhead and utilizing the sensed power to control the operation of a second motor which drives the concrete conveyor.
Electrical control systems have been developed to replace the hydraulic and mechanical controls disclosed in Fosse U.S. Pat. No. 4,340,553. Examples of electrical systems for controlling the concrete feed in packerhead pipe making machines are disclosed in Hand U.S. Pat. Nos. 4,336,013 and 4,406,605. These electrical control systems do not modulate the lift rate of the packerhead to self-compensate for overpack and underpack conditions which can occur during the formation of a concrete pipe in a packerhead machine. It is thus apparent that the need exists for a combined concrete feed and packerhead lift control system to compensate for overpack and underpack conditions which can otherwise occur along the sidewalls of a concrete pipe formed on packerhead machines available in the prior art.